An ionic compound generally forms crystals in which positively charged cations and negatively charged anions pull electrostatically against each other. When this ionic compound is dissolved in various other liquids, including water, it provides a liquid that carries electricity; that is, an electrolyte solution. Electrolyte solutions obtained by dissolving an ionic compound in an organic solvent are commonly used in, for example, nonaqueous electrolyte batteries and capacitors.
Some ionic compounds, when the temperature is raised, undergo activation of thermal motion to such an extent as to overcome the ionic interactions, causing the compound itself to become liquid and capable of carrying electricity. A salt in such a state is generally referred to as a “molten salt.”
The chemical species present in the molten salt are all charged cations or anions; no neutral atoms or molecules are present. Therefore, elements which cannot be obtained from an aqueous electrolyte solution because they have too large a reducing or oxidizing power with respect to water, including metals such as alkali metals, aluminum and rare-earth elements, and non-metals such as fluorine, can be electrolyzed in a molten salt and obtained in elemental form. This has become a main industrial application of molten salts.
Some such molten salts maintain a liquid state at room temperature and do not solidify even at very low temperatures. Such molten salts which maintain a liquid state at room temperature or lower are referred to in particular as “room-temperature molten salts” or “ionic liquids.” To minimize electrostatic interactions between the cations and anions which make up the ionic liquid, either or both are molecular ions of a substantial size, and are moreover monovalent to minimize the charge and electrostatic interactions.
Research is actively being pursued on applications for such ionic liquids in electrolytic deposition and in electrolytes for batteries and other purposes. However, because ionic liquids generally have a high moisture absorption and are difficult to handle in air, such applications has yet to be fully realized.
In light of the above, the 1-ethyl-3-methylimidazolium tetrafluoroborate reported by Wilkes et al. in 1992 is a remarkable ionic liquid that can be handled even in air. This new ionic liquid led to the synthesis of many ionic liquids which are combinations of numerous alkylimidazolium cations having different side chains with various anions. Although the properties and applications for these ionic liquids are being actively investigated, there exists a desire for the development of various ionic liquids that can be more conveniently produced and are easy to handle.
Nonaqueous liquid electrolyte-type electrical double-layer capacitors can be charged and discharged at a high current, and thus hold considerable promise as energy storage devices for such applications as electrical cars and auxiliary power supplies.
Prior-art nonaqueous liquid electrolyte-type electrical double-layer capacitors are constructed of positive and negative polarizable electrodes made primarily of a carbonaceous material such as activated carbon and a nonaqueous electrolyte solution. The composition of the nonaqueous electrolyte solution is known to have a large influence on the withstand voltage and electrostatic capacitance of the capacitor.
The nonaqueous electrolyte solution is composed of an electrolyte salt and a nonaqueous organic solvent. Studies have been conducted on various combinations of such electrolyte salts and nonaqueous organic solvents.
For example, quaternary ammonium salts (e.g., JP-A 61-32509, JP-A 63-173312, JP-A 10-55717) and quaternary phosphonium salts (e.g., JP-A 62-252927) are commonly used as the electrolyte salt because of their solubility and degree of dissociation in organic solvents, as well as their broad electrochemical stability range. Organic solvents that are commonly used on account of their high dielectric constant, broad electrochemical stability range and high boiling point include ethylene carbonate, diethyl carbonate, propylene carbonate, butylene carbonate, γ-butyrolactone, acetonitrile and sulfolane.
However, in nonaqueous electrolyte-type electrical double-layer capacitors currently in use, the inadequate solubility of electrolyte salts (e.g., quaternary ammonium salts, quaternary phosphonium salts) in organic solvents commonly used for this purpose limits the amount of salt that can be added, resulting in nonaqueous electrolyte solutions of lower ionic conductivity and electrical double-layer capacitors of lower electrostatic capacitance.
Moreover, because the electrolyte salts have a low solubility, they tend to crystallize at low temperatures, compromising the low-temperature characteristics of the electrical double-layer capacitor.
In light of these circumstances, the objects of the invention are to provide ionic liquids which can be easily and efficiently produced, electrolyte salts for electrical storage devices which have excellent solubility in organic solvents for nonaqueous electrolyte solutions and have a low melting point, liquid electrolytes for electrical storage devices which include these electrolyte salts, and also electrical double-layer capacitors and secondary batteries of excellent low-temperature properties which are constructed using such liquid electrolytes.
We have conducted extensive investigations aimed at achieving the above objects, as a result of which we have discovered that some quaternary ammonium salts and quaternary phosphonium salts bearing at least one alkoxyalkyl substituent have low melting points and excellent characteristics as ionic liquids.
Moreover, we have found that, because quaternary ammonium salts and quaternary phosphonium salts bearing at least one alkoxyalkyl substituent have excellent solubility in nonaqueous organic solvents used in electrical storage devices and also have a low melting point, liquid electrolytes prepared using such quaternary salts can be obtained to a higher concentration than previously possible and are less likely to result in deposition of the electrolyte salt at low temperatures. We have also found that electrical double-layer capacitors manufactured using such liquid electrolytes have a high electrostatic capacitance and excellent low-temperature characteristics.
Accordingly, the present invention provides the following.    (1) An ionic liquid characterized by having general formula (1) below and a melting point of up to 50° C.
wherein R1 to R4 are each independently an alkyl of 1 to 5 carbons or an alkoxyalkyl of the formula R′—O—(CH2)n—, R′ being methyl or ethyl and the letter n being an integer from 1 to 4, and any two from among R1, R2, R3 and R4 may together form a ring, with the proviso that at least one of groups R1 to R4 is an alkoxyalkyl of the above formula; X is a nitrogen or phosphorus atom; and Y is a monovalent anion.    (2) The ionic liquid of (1) above which is characterized by having a melting point of up to 25° C.    (3) The ionic liquid of (1) or (2) above which is characterized in that X is a nitrogen atom.    (4) The ionic liquid of (3) above which is characterized in that X is a nitrogen atom, R′ is methyl, and the letter n is 2.    (5) The ionic liquid of (1) or (2) above which is characterized by having general formula (2) below
wherein R′ is methyl or ethyl, X is a nitrogen or phosphorus atom, Y is a monovalent anion, Me signifies methyl and Et signifies ethyl.    (6) The ionic liquid of any one of (1) to (5) above which is characterized in that Y is BF4−, PF6−, (CF3SO2)2N−, CF3SO3− or CF3CO2−.    (7) The ionic liquid of (5) above which is characterized by having general formula (3) below
wherein Me signifies methyl and Et signifies ethyl.    (8) An electrolyte salt for electrical storage devices, which salt is characterized by being a quaternary salt of general formula (1) below
wherein R1 to R4 are each independently an alkyl of 1 to 5 carbons or an alkoxyalkyl of the formula R′—O—(CH2)n—, R′ being methyl or ethyl and the letter n being an integer from 1 to 4, and any two from among R1, R2, R3 and R4 may together form a ring, with the proviso that at least one of groups R1 to R4 is an alkoxyalkyl of the above formula; X is a nitrogen or phosphorus atom; and Y is a monovalent anion.    (9) The electrolyte salt for electrical storage devices of (8) above which is characterized by being a quaternary salt in which X is a nitrogen atom.    (10) The electrolyte salt for electrical storage devices of (9) above which is characterized by being a quaternary salt in which X is a nitrogen atom, R′ is methyl and the letter n is 2.    (11) The electrolyte salt for electrical storage devices of (8) above which is characterized by being a quaternary salt having general formula (2) below
wherein R′ is methyl or ethyl, X is a nitrogen or phosphorus atom, Y is a monovalent anion, Me signifies methyl and Et signifies ethyl.    (12) The electrolyte salt for electrical storage devices of any one of (8) to (11) above which is characterized in that Y is BF4−, PF6−, (CF3SO2)2N−, CF3SO3− or CF3CO2−.    (13) The electrolyte salt for electrical storage devices of (11) above which is characterized by having general formula (3) below
wherein Me signifies methyl and Et signifies ethyl.    (14) The electrolyte salt for electrical storage devices of any one of (8) to (13) above which is characterized by having a melting point of up to 25° C.    (15) A liquid electrolyte for electrical storage devices which is characterized by being composed solely of the ionic liquid of any one of (1) to (7) above.    (16) A liquid electrolyte for electrical storage devices which is characterized by being composed solely of the electrolyte salt for electrical storage devices of (14) above.    (17) A liquid electrolyte for electrical storage devices which is characterized by including at least one ionic liquid of any one of (1) to (7) above and a nonaqueous organic solvent.    (18) A liquid electrolyte for electrical storage devices which is characterized by including at least one electrolyte salt for electrical storage devices according to any one of (8) to (13) above and a nonaqueous organic solvent.    (19) The liquid electrolyte for electrical storage devices of (17) or (18) above which is characterized in that the nonaqueous organic solvent is a mixed solvent which includes as a main component ethylene carbonate or propylene carbonate.    (20) The liquid electrolyte for electrical storage devices of (17) or (18) above which is characterized in that the nonaqueous organic solvent is one selected from among ethylene carbonate, propylene carbonate, vinylene carbonate, dimethyl carbonate, ethyl methyl carbonate and diethyl carbonate, or a mixed solvent of two or more thereof.    (21) An electrical double-layer capacitor having a pair of polarizable electrodes, a separator between the polarizable electrodes and a liquid electrolyte, which electrical double-layer capacitor is characterized in that the liquid electrolyte is a liquid electrolyte for electrical storage devices according to any one of (15) to (20) above.    (22) The electrical double-layer capacitor of (21) above which is characterized in that the polarizable electrodes include as a main component a carbonaceous material prepared from a resin.    (23) The electrical double-layer capacitor of (22) above which is characterized in that the resin is a phenolic resin or a polycarbodiimide resin.    (24) The electrical double-layer capacitor of (22) above which is characterized in that the carbonaceous material is prepared by carbonizing a phenolic resin or polycarbodiimide resin, then activating the carbonized resin.    (25) An electrolyte solution for electrical storage devices which is characterized by being composed of the liquid electrolyte for electrical storage devices of (15) or (16) above and an ion-conductive salt which is solid at ambient temperature.    (26) The electrolyte solution for electrical storage devices of (25) above which is characterized in that the ion-conductive salt is a lithium salt.    (27) The electrolyte solution for electrical storage devices of (25) or (26) above which is characterized by including also a nonaqueous organic solvent.    (28) A secondary battery having a positive electrode and a negative electrode, a separator between the positive and negative electrodes, and an electrolyte solution, which secondary battery is characterized in that the electrolyte solution is an electrolyte solution for electrical storage devices according to any one of (25) to (27) above.    (29) An electrical double-layer capacitor having a pair of polarizable electrodes, a separator between the polarizable electrodes and a liquid electrolyte, which electrical double-layer capacitor is characterized in that the liquid electrolyte is an electrolyte solution for electrical storage devices according to any one of (25) to (27) above.    (30) An electrical storage device having a positive electrode and a negative electrode, a separator between the positive and negative electrodes, and a liquid electrolyte, which electrical storage device is characterized in that the positive electrode is activated carbon, the negative electrode is a carbonaceous material that is capable of occluding and releasing lithium ions, and the liquid electrolyte is an electrolyte solution for electrical storage devices according to any one of (25) to (27) above.